As recent development of a communication technology, a satellite communication system can provide diverse services such as video and audio services to a moving user terminal through a satellite. A repeater for reducing dead zones in the satellite communication system is realized on ground.
Satellite Digital Multimedia Broadcasting (DMB) and European Satellite-Digital Multimedia Broadcasting (S-DMB) are representatives as the satellite communication system having a repeater. The repeater is defined as a gap filler in a Korean satellite DMB method and defined as Intermediate Module Repeater (IMR) in a European S-DMB method. A repeater realized in a current satellite communication system includes a simple amplification repeater for amplifying the signal transmitted from the satellite at the same band, and repeating the signal to a user terminal and a frequency conversion repeater for frequency-converting the signal transmitted from the satellite and repeating the signal to the user terminal. In addition, a repeater with a diverse signals processing function as well as the frequency converting function has been developed.
In the satellite communication system described above, the user terminal can receive the signal directly from the satellite or indirectly through the repeater. Since the signal directly transmitted from the satellite and the signal indirectly transmitted through the repeater go through different channel environments, the user terminal can improve receiving performance by applying a diversity technology for simultaneously receiving a plurality of signals transmitted from the satellite and the repeaters.
Conventional diversity technologies realized to be applicable in a general mobile communication system include cooperative relay transmission technology using a repeater and cooperation diversity technology based on presence of errors. Each prior art will be described in detail.
In the cooperative relay transmission technology using the conventional repeater, each terminal functions as a repeater in signal transmission. Therefore, the cooperative relay transmission technology relates to a relay transmission technology for acquiring a diversity gain by receiving a signal from a terminal and a base station. That is, in the conventional cooperative relay transmission technology, since relay stations receive and retransmit an up-link signal of a source terminal, a transmission diversity gain can be acquired in up-link transmission of the terminal having a single antenna. The conventional cooperative relay transmission technology can improve reliability in reception signal processing of the base station as well as the diversity gain by optimally maintaining the number of relay terminals to a specific source terminal by controlling channel quality between the base station and the relay station for determining the relay terminal or distance threshold. The cooperation diversity technology based on presence of errors relates to a diversity method for relaying only a proper signal by checking the presence of errors in the signal received in the repeater. That is, in case of relay of a Multiple-Input Multiple-Output (MIMO) wireless network, the cooperation diversity technology selectively relays only data of no error by checking whether there is any error after decoding the data transmitted from a source according to each stream.
In a conventional diversity method introduced by “Mohammad Janani” as the conventional diversity method using the error correction code, data of an opposite repeater are transmitted between repeaters based on an error correcting code such that a receiver can obtain the diversity gain. However, there is a problem that this method may generate a loss by performing selective transmission after checking whether an error occurs in an opposite repeater.
Especially, most of the conventional cooperative diversity technologies for the terrestrial mobile communications including the above mentioned conventional scheme are based on time division duplexing (TDD) mode, and this is impractical to the satellite communication system.